1. Field of the Invention
This invention relates to a bidirectional switch composed of two MOS transistors having a trench type gate structure and sharing a common drain, specifically to the bidirectional switch with a reduced ON resistance.
2. Description of the Related Art
In the case where a secondary battery such as a lithium ion battery is used to provide a load with electric charges stored in it as an electric current, preventing overdischarging as well as re-charging the lithium ion battery or the like is required when discharging has proceeded to a certain extent. Also, it is necessary to control the charging so that the lithium ion battery or the like is not overcharged.
Thus, in order to control the charging and the discharging of the lithium ion battery or the like, there is a need for a bidirectional switch that controls a charging current as well as a discharging current that is completely opposite in a direction of a current flow. Two discrete MOS transistors with their drain electrodes connected with each other are used as the bidirectional switch in the beginning.
In this case, one of source electrodes is connected to a cathode or the like of the lithium ion battery or the like and another source electrode is connected to the load or a battery charger, and electric potentials at the gate electrodes and electric potentials at the source electrodes and the like of the two MOS transistors are controlled by a control IC so that the charging/discharging current of the lithium ion battery or the like is controlled through operations of the bidirectional switching.
However, as portable terminal equipment comes into widespread use, various kinds of components are required to be reduced in their sizes. As the bidirectional switch controlling the charging/discharging current of the lithium ion battery or the like is also required to be more compact, a single bidirectional switch that is composed of two MOS transistors integrated into a single semiconductor die is developed. As an example of the single bidirectional switch, a bidirectional switch with a planar type gate structure is disclosed in Japanese Patent Application Publication No. H11-224950.
However, there is a problem that reducing a cell size and reducing the ON resistance is difficult with the planar type gate structure, because a large area is required to secure a source-drain dielectric breakdown voltage BVDS and because a gate electrode is formed in a horizontal direction on the semiconductor die. On the other hand, Japanese Patent Application Publication Nos. 2004-274039 and 2002-118258 disclose attempts to reduce a pattern size and the ON resistance by adopting a trench type gate structure in which the gate electrode is formed in a vertical direction in the semiconductor die.
FIG. 6 shows a cross-sectional view of a bidirectional switch having the trench type gate. An N type well layer 52 is formed on a P type semiconductor substrate 51. A P type body layer 53 is formed on the N type well layer 52, and a trench 54 extending from a surface of the P type body layer 53 into the N type well layer 52 is formed.
A first gate electrode 56a and a second gate electrode 56b are formed in the trench 54 so that each of them extends from each of both sidewalls to a bottom surface of the trench 54 through a gate insulation film 55. A region in the trench 54 interposed between the first gate electrode 56a and the second gate electrode 56b is filled with an insulation film 64 and its surface is planarized. The N type well layer 52 extending from the both sidewalls to the bottom surface of the trench 54 through the gate insulation film 55 forms N type common drain layers 65a and 65b of both MOS transistors.
The N type drain layer 65a is the N type well layer 52 in a portion between edges of the first gate electrode 56a and the second gate electrode 56b in the bottom surface of the trench 54. Each of the N type drain layers 65b is the N type well layer 52 in a portion extending from each of the sidewalls of the trench 54 to the edge of each of the first and second gate electrodes 56a and 56b in the bottom surface of the trench 54, respectively.
A first N+ type source layer 57 is formed in the P type body layer 53 on one side of the trench 54, and a second N+ type source layer 58 is formed in the P type body layer 53 on another side of the trench 54. A first P+ type contact layer 59 connected to the P type body layer 53 is formed in the first N+ type source layer 57, and a second P+ type contact layer 60 connected to the P type body layer 53 is formed in the second N+ type source layer 58.
An interlayer insulation film 61 is formed on the first and second N+ type source layers 57 and 58 and the like. There are formed a first source electrode 62 connected with the first N+ type source layer 57 and the like through a contact hole formed in the interlayer insulation film 61 and a second source electrode 63 connected with the second N+ type source layer 58 and the like through a contact hole formed in the interlayer insulation film 61. The first and second gate electrodes 56a and 56b are also drawn out onto the interlayer insulation film 61 through contact holes (not shown) formed in the interlayer insulation film 61.
FIGS. 7A and 7B are equivalent circuit diagrams of the bidirectional switch. The bidirectional switch is composed of two MOS transistors sharing a common drain layer D. FIG. 7A shows an electric potential at each of electrodes of the bidirectional switch in an ON state. A high voltage VH is applied to a first source electrode S1, while a low voltage VL is applied to a second source electrode S2.
When symbols in FIG. 7A are compared with symbols in FIG. 6, S1 corresponds to the first source electrode 62, S2 corresponds to the second source electrode 63, D corresponds to the common drain layers 65a and 65b, G1 corresponds to the first gate electrode 56a, and G2 corresponds to the second gate electrode 56b. The symbols in FIG. 6 are used in the following explanations.
An N type channel layer (not shown) is formed in a surface of the P type body layer 53 facing each of the gate electrodes 56a and 56b through the gate insulation film 55 by applying a voltage equal to or higher than VH+Vt (threshold voltage) to the first gate electrode 56a and a voltage equal to or higher than VL+Vt to the second gate electrode 56b. 
As a result, an ON current flows from the first source electrode 62 at a high electric potential to the common drain layers 65a and 65b through the channel layer on a side of the first source electrode 62. The ON current flown into the common drain layers 65a and 65b further flows into the second source electrode 63 at a low electric potential through the channel formed in the P type body layer 53 on a side of the second source electrode 63. That is, there is formed a current path from the first source electrode 62 to the second source electrode 63.
On the other hand, a current path from the second source electrode 63 to the first source electrode 62 is formed by applying the high voltage VH to the second source electrode 63 and the low voltage VL to the first source electrode 62 and an appropriate voltage to each of the gate electrodes 56a and 56b. That is, the bidirectional switching operations can be implemented by setting the appropriate voltage to be applied to each of the electrodes.
FIG. 7B shows voltages applied to the electrodes when the current flowing through the bidirectional switch is turned off and the bidirectional switch is put into an OFF state. The voltage applied to the first gate electrode 56a on the side of the first source electrode 62 at the high voltage VH is reduced from VH+Vt to VH, while the voltage applied to the second gate electrode 56b on the side of the second source electrode 63 at the low voltage VL is reduced from VL+Vt to VL.
As a result, the both channel layers are eliminated and the ON current is cut off to put the bidirectional switch into the OFF state. In this case, a current path through parasitic diodes formed between the P type body layer 53 and the N type well layer 52 in the two MOS transistors constituting the bidirectional switch is also cut off because the parasitic diode to which the low voltage VL is applied is reverse-biased.
Since the bidirectional switch described above adopts the trench type gate structure, it is possible to reduce the pattern size. Also, it has high current drive capability to reduce the ON resistance in the ON state. However, it is not sufficient to satisfy requirements on the bidirectional switch to further reduce the ON resistance as the equipment is further reduced in size. Further reduction of the ON resistance in the ON state of the bidirectional switch is required.